The prior art teaches a number of ways of automatically leveling the main frame of a construction machine during its advance along a grade, at a pre-determined and accurately held plane and height regardless of variations in the grade, slope or height of the ground being traversed. The prior art also teaches a number of ways of automatically leveling the working tool of a construction machine under the foregoing conditions. The means by which these results are accomplished and the accuracy of the end product of course vary as much as the number of teachings. The U.S. Pat. Nos. 3,249,026, 3,280,846, 3,292,511, 2,761,666, 3,710,695 and 3,606,827 are exemplary of these teachings and each has its advantages and disadvantages.
One of the problems facing the construction machine art has been the accurate control of the slope and grade of a working tool as the machine negotiates a path of travel over a rough grade. Various forms of suspension have been used between the working tool and the main frame of the machine or between the main frame, carrying a fixed tool, and the ground engaging means used to transport the machine. Machines of this type employ working tools that extend transversely of the main frame either in front of the frame, at a mid-point of the frame or at the rear of the frame. If the frame is adjusted in relation to the wheels or tractors and the tool is fixed to the frame then the location and number of supporting wheels and frame adjusting points become critical to the accurate control of the system.
Generally it is easier to maintain three suspension points in a plane than it is to maintain four suspension points in a plane. Attempts have been made to use large flexible frames to carry rigidly attached working tools and support the four corners of the frame on adjustable suspension means over a wheel or tractor at each corner. In this arrangement a grade reference on each side of the machine is required and the slope is determined by the co-planar relationship of the grade reference.
In order to eliminate one grade line, which has several known advantages, it is necessary to control the pair of suspension means on that side in a manner to maintain the required slope or level of the working tool. This is done by operating that pair of suspension means simultaneously from a gravity operated sensor to raise and lower that side of the frame.
The prior art machines fall into a number of categories: (a) those that employ large flexible frames and four corner supports between the frame and the ground engaging means, (b) those that employ rigid frames and either three or four supports between the frame and the ground engaging means, and (c) those that employ either flexible or rigid frames and adjustably support the tool in relation to the frame, using relatively rigid supports between the frame and the ground engaging means. It is apparent that the larger heavy machines, such as those which span one or more lanes of a highway, are easier to control and produce an end product meeting the accepted standards of grade and slope than smaller machines which because of their dimensions are subject to greater deviations in negotiating and correcting for given changes in grade, slope or level. In both large and small machines the working tool can be carried transverse the frame or longitudinal of the frame at various locations and the tool can be on either side of the main frame or in the so-called straddle position. Some machines are versatile enough to tolerate any type of tool mounting.
The art recognizes that the problems associated with frame and tool adjustment to control grade and slope for these purposes are not related, and the teachings from one art are not necessarily applicable to another. Likewise the suspensions used for a transverse tool do not translate into something useful for a side-mounted working tool. Thus a side-mounted tool such as illustrated by the Cheney U.S. Pat. No. 3,292,511 requires a vertical adjustment for height or grade control, a gravity responsive tilt correction and a pitch or grade correction. In the Cheney device, a change in pitch influences the height of the tool and a change in tilt.
Such construction machines include as their basic parts a main frame used to support one or more working tools; ground engagement or traction means such as wheels, skids or endless tracks, and various kinds of adjustable support means between the main frame and the ground engagement means or between the main frame and the working tool. The ground engagement means are in direct rolling or sliding contact with grade (the elevation), slope (the inclination); and the surface conditions encountered differ widely. Their common objective is to utilize adjustable frame or tool support means to maintain the frame and tool at a predetermined plane reflecting as near as possible a desired grade and slope irrespective of the irregularities of the ground over which the machine passes.